This invention relates to acoustic surface wave devices and, more particularly, is concerned with acoustic surface wave filters.
Surface acoustic waves devices, or SAW devices, formed of a planar piezo-electric crystal structure have been utilized as filters. The piezo-electric materials have pairs of interleaved conductive fingers on their surface, which launch acoustic waves in response to electrical inputs and generate electrical signals in response to travelling acoustic surface waves. An input, or transmitting, transducer launches the acoustic surface waves along a predetermined path on the surface of the substrate. An output, or receiving, transducer detects the acoustic waves and generates electrical signals in response to them.
This basic type of device may be used as a relatively broadband filter. The frequency of operation of SAW devices is a function of the shape and size of the transducers. Because these electro-acoustical transducers produce electrical signals in response to surface acoustical waves, this type of device may be used as a relatively broadband filter. Since the electro-acoustical transducers have a high insertion losses at frequencies outside the band of frequencies that is defined by the transducer geometry, the transducer will operate like a bandpass filter, the center frequency of which is a function of the spacing between pairs of transducer fingers.
SAW-type filters have also been developed for use in applications where a relatively narrow bandwidth is desired. These rely on resonance which is set up by reflectors on opposite sides of both the input and the output transducers so that the surface acoustic waves travelling toward the reflectors are strongly reflected back in the opposite direction to maintain resonance. Examples of SAW resonance filters are described in the literature in the following articles:
"Crossed-Resonator SAW Filter: A Temperature-Stable Wider-Band Filter on Quartz" by R. L. Rosenberg and L. A. Coldren, IEEE 1979 Ultrasonics Symposium Proceedings, pages 836-837.
"The Temperature Dependence of SAW Resonator Filters Using Folded Acoustic Coupling" by L. A. Coldren, IEEE 1979 Ultrasonics Symposium Proceedings, page 830.
"Reflection-Dependent Coupling Between Grating Resonators" by R. L. Rosenberg and L. A. Coldren, IEEE 1976 Ultrasonics Symposium Proceedings, pages 281-285.
"Acoustically Cascaded ASW Resonator--Filters" by P. S. Cross, R. V. Schmidt, and H. A. Haus, IEEE 1976 Ultrasonics Symposium Proceedings, pages 277-280.
"Narrow-Band SAW Filters Using stepped-Resonators with Tapered Gratings" by P. J. Edmonson, C. K. Campbell and P. M. Smith, IEEE 1984 Ultrasonics Symposium Proceedings, pages 235-238.
Another example of a SAW resonance filter is the multisection staggered resonator filter. In this type of filter there are a number of channels which are arranged so that both the input and output transducers have a reflectors on both sides of them along a linear path. There are two or more adjacent channels, each of which is constructed with a slightly different separation distance between the transducer and the reflectors. This difference in spacing is typically much less than a wavelength of the SAW on the substrate. These staggered resonator filter channels provide a passband which is determined by the overlapping passbands of the individual channels. The staggered resonator filter, therefore, provides a broader passband than a single channel which is of importance in applications where it is desired to extend SAW filter technology to passbands that lie between the narrow passband of a single stage resonator filter, and a broadband simple inline input/output transducer filter. The staggered resonator filter, while providing a wider passband than a single resonator filter, generally has an out-of-band rejection which is generally equal to, or even slightly less, than the out-of-band rejection for a single channel resonator filter.
The present invention is directed to a staggered resonator filter that has multiple channels, wherein the multiple channels are provided in pairs, and the output transducer for one channel of each pair is configurated so that the signal sensed in one channel is one-half wavelength displaced from the signal sensed in the other channel. Due to this differential detection by the output transducers, out-of-band signals will cancel each other, and a very large out-of-band rejection is obtained. The in-band signal, because of the amplitude versus frequency and phase versus frequency characteristics of the signals detected by the output transducer, are combined to provide a passband which is broader than a single resonator channel.